(Model) Helicopter Physics

If you’ve ever wondered how a helicopter is able to fly, or would just like to see some awesome RC piloting, the four videos after the break should be just the thing! Although the basic physics of how one works is explained in the last three, one would still be hard pressed to explain how [Carl] is able to fly his RC helo the way he does. The video has to be seen to be believed or even explained, but one of the simpler tricks involved taking off a few feet, doing a forward flip, and flying off backwards and upside-down!

As explained in detail in the othervideos, a helicopter is controlled by something called a swash plate on the main rotor, which in short translates a linear action into a rotational one. The same thing is done with the tail rotor, but you’ll have to check out the videos after the break for a full explanation! Really ingenious that someone could come up with this analog control system to use before computers were available.

Of particular interest to physics geeks, an explanation of gyroscopic precession is given in the fourth video. Controlling a helicopter may not work exactly the way you thought!

Honestly, it never occurred to me that they could vary the pitch of the rotor in different arcs of the rotation. I assumed that maneuvering was accomplished by the tail rotor and some sort of control surfaces like airplanes have.

Indeed! These model helicopters are very, very “real”, with all the basic controls of their full scale relatives along with mind boggling performance.

Learning to fly these helicopters requires serious mental gymnastics – constantly balancing collective, 2 axes of cyclic, and tail rotor to maintain control of the inherently unstable craft. The first time I hovered, I was some completely occupied with the task that I was literally unable to speak.

Anyway, they are very cool machines. Electric power is popular, but there are also gas and real turbine engines as well.

Crazy stuff. Excellent videos and explanation. In fact in front of my office there are a few guys who fly those helis like that (and worse) and I always wondered with a friend how force vectors apply to those helis to keep them afloat. Funnier, when my son (5) saw these guys flying he asked me “Dada, do these guys really know how to fly a helicopter?” (because they looked so erratic) – “Oh yes, sweetie, these guys do!” Very cool!

You don’t need to do hard 3d, which I agree doesn’t look all that great no matter how impressive it is. Smooth, large air, advanced aerobatics though… I definitely see the attraction. Check this video out:

they could have added an awesome dubstep song to this vid. I am definitely going to see if my tiny dual rotor heli can do this and if not get one that can. props to all the guys that can do this frikkin awesome!!!

It’s also interesting to note that any control input on a helicopter means that all the others have to be adjusted.

Take the last video, where you want to move forward.

(1) You tilt the lift vector forward (by generating more lift on one side), but now the vertical component of the lift vector is reduced.

(2) Therefore you have to increase the collective pitch to stop the helicopter falling. More collective pitch means that the torque applied to the rotor increases.

(3) To cancel out that torque you need to increase the pitch of the tail rotor, but now there’s more force pushing the helicopter sideways.

(4) To compensate for the extra sideways force you need to use the main rotor to generate an opposing force (by tilting it sideways). This again tilts the lift vector and requires more collective pitch to compensate, so go back to (2) and repeat the whole thing.

Most RC helicopters have at least a single-axis gyroscope that handles (3) so the pilot can ignore it. More advanced ones have triple-axis gyroscopes that effectively provide a low-pass filtering effect, since the dynamics of a small helicopter are extremely quick (helicopters that don’t have the extra gyroscopes use a mechanical low-pass filter instead).

There are all sorts of un-intuitive effects, and getting one wrong can easily result in a crash. For example, if a helicopter is descending vertically, in some cases the only way to stop this is to move forward – even the maximum collective pitch will not be adequate to halt the fall. It’s due to vortices that get generated around the rotor in vertical flight; moving forward (or sideways, or backwards) gets the rotor into clean air and allows it to generate lift much more efficiently.

Regarding realism – you can get RC helicopters that look and act very much like the real thing (even powered by real turbines, as was mentioned above). However, these tend to be ‘boring’ to watch compared to the aggressive acrobatic ones.

I grew up with verti-birds, which were really cool… and easy to fly. I’ve always wanted to make a giant one, but I digress…

In the early 1980’s I borrowed a library book on how to fly real helicopters. It was an old text and focused entirely on piston powered helicopters.

Holy cow! In addition to everything you mentioned, there was a bunch about monitoring the engine manifold vacuum. That is critical to know the load of the engine. When collective pitch is added to generate more lift, it increases the load on the engine. So you need to increase throttle. If you overload the engine, bad stuff happens. And you also need to monitor engine rpm.

In addition to that, I’ve heard that the old Sikorskys in particular have a seriously heavy collective (you need to constantly hold the collective up to keep the helicopter flying).

The modern turbine helicopters use governors for the engine, as do many of the newer piston ones. That would certainly help to reduce pilot workload.

In the little RC helicopters it doesn’t really matter – the mechanics and motor can handle plenty of abuse in the form of going over/under recommended RPM, and the motor has plenty spare power if you do get into strife.

The same is not true of full size helicopters, where the space between “minimum rotor/engine RPM” and “maximum rotor/engine RPM” is very small and the power available is often not very much different to the power required to fly.

Even the small ones without the electronic correction (FBL), usually have a flybar which helps the heli maintain attitude through mechanical means. Very few can perform advanced maneuvers on a heli with neither a flybar nor a flybarless controller.

ive yet to be able to fly my heli for any length of time without seriously damaging the thing. so far ive been able to repair it well enough to fly again. but im seriously thinking about dropping an imu in there and completely idiot proofing it. no level of understanding about how the physics and mechanics work will help me be any better at flying the damn thing.

i completely agree with that. my new favorite heli is the mcp-x. quick, accurate, and cheap to maintain.

i also have a blade 400, but that averages about $50 in damage per crash, which is roughly every 6 flights. it’s a awe-inspiring experience to get it flying, and just as interesting to repair it.

with the mcp-x, as long as you fly it over grass, and make a effort to reduce, or cut the throttle before it hits the ground, is normally fine. pick it up, start over. big wear items are the rotor-head linkages, main gear, and the poor hollow cf tail boom– the typical upgrade is a solid cf version.

compared to my b400– one crash will typically damage the blades, flybar, feathering shaft, main shaft, some of the linkages between the swashplate, and sometimes servo gears.

my total replacement cost since buying the mcp-x is less then $30. total replacement cost of my b400 is currently over $400.

I have seen an Apache attack heli do a complete barrel roll, as the Army was beginning to field them back in the 80’s. The test pilot at the controls was doing demo flights for the several aviation units on my base. SOME full scale heli’s have capabilities I’d rather not be a participant of.

As to inverted flight with a semi articulated or full articulated main rotor, yes, it can be done. Depends entirely on how the controls and fuel/lubrication systems are set up. It requires VERY careful planning of the pilot before attempting.

And now someone HAS to do it on a REAL helicopter! Of course setting the whole thing up so it can be remotely controlled, because the “meat servo”. But if someone would do it, and do nearly the same things (I’m not sure, but I think that they couldn’t really do it on a real one, because the mass of the real helicopter…) that would be really mind-blowing…

PS, correct me if you think that it could be done just like on the RC helicopter…

One thing that tends to affect real helicopters more than model ones is the flexibility of the blades. On the model ones the blades are more or less rigid, and if the head is rigid too then there’s very little chance of the blades hitting the tail boom.

On a full size helicopter, the blades are pretty bendy and (as was mentioned above) they’re also hinged to allow free movement up/down. The only thing keeping them fairly straight is centrifugal force.

If you go from positive collective (blades pulling upwards) to negative collective (blades pushing downwards, as needed for flying upside down) quickly then you’ll most likely end up running the blades into the tail boom, shortly followed by running the helicopter into the ground.

As far as I know, you can’t actually do this because no full size helicopter allows negative collective pitch. You can still do very short inverted flights (ie loops that are always at positive collective) but not sustained inverted flight.

I flew R/C helicopters in the early 80’s. We didn’t even think of flying upside down. I was just pleased to get into hover, fly around, get back into hover and land without having to make new rotor blades.